Serum lipoprotein-X was investigated in 12 patients with cholestasis in basal conditions and 20 minutes after intravenous heparin. In all cases, lipoprotein-X was positive in the first sample and became negative after heparin. A decrease in triglycérides was observed in all patients after heparin with an increase in free fatty acids and mobilization of prebetalipoproteins, and a transformation of lecithin into lysolecithin in 4 cases after heparin. These facts suggest that heparin plays an important role in the mobilization of serum lipoprotein-X. © 1975.